In recent years, color ink-jet printers have gained many advanced functions although their prices have lowered. Also, color laser beam printers (color LBPs) have become prevalent, and many color printing systems have been provided. The print resolution is normally 600 DPI, but is projected to soon become 1200 DPI. On the other hand, as for tone expression, since it is difficult for a printer engine to attain tone expression of 4 bits (16 gradation levels) or more at the aforementioned resolution, smooth tone expression is attained using about 200 lines per inch (1 pi) by gradating such as dithering or the like.
However, it is difficult for an electrophotographic color print system to stably maintain high print quality since complicated exposure, developing, transfer, and fixing processes are required. Especially, when a non-contact developing scheme is adopted to use an inexpensive photosensitive body or developing device, the following problems are posed. Note that a detailed mechanism is disclosed in Japanese Patent Laid-Open No. 08-337007.
(1) In case of image data formed by arranging patches of different colors (especially, primary colors such as yellow (Y), magenta (M), cyan (C), black (K), and the like) each having a predetermined size, a gap is formed at the boundary between neighboring color patches, and appears as if a white line were present there. This phenomenon is called a “white gap”.
More specifically, even when a print result shown in FIG. 24 is to be obtained, white gaps form at color boundaries, as shown in FIG. 23. A white gap is similarly formed at the boundary between neighboring colors in addition to such pattern.
When a white gap is generated, a white line, which is not present on image data, appears on a natural image, graph chart, color text in a uniform background, and the like, thus considerably deteriorating image quality.
Such phenomenon is caused by potential contrast, a change in developing characteristics of a developing agent, and a developing scheme. Note that the potential contrast is produced by a latent image potential (dark potential, bright potential) on a photosensitive drum surface where an image is formed, and a developing potential. Also, the developing characteristics of a developing agent change due to changes in environment or durability along with an elapse of time.
Such phenomenon will be explained using linear image data. FIG. 25A exemplifies the state of the surface potential on the photosensitive body. In the example shown in FIG. 25A, the potential of a print region is set at −100 V, and that of a non-print region is set at −700 V, and the developing bias potential is set at an intermediate value. Note that white gaps are highly likely to be generated at portions A and B in FIGS. 25A and 25B. This is because white gaps readily form due to a toner sweep-up phenomenon when the density potential of toner changes abruptly and one lower density level is completely OFF.
There is another origin for generating white gap. Although exposing corresponding to cyan image and black image with a laser as shown in FIG. 27A, a certain gradient shown in FIG. 27B occurs in a portion where the latent image potential changes. Developing this portion, since a portion of which latent image potential is lower than a threshold cannot be developed, a gap is formed at the boundary between the cyan and black as shown in FIG. 27C, and appears a “white gap”.
The present invention has been made in consideration of the conventional problems.